Add multi-arch build support and detailed antenna guide

Multi-arch Docker builds:
- build-multiarch.sh: Cross-compile amd64+arm64 on x64 and push to
  registry, so RPi5 can docker pull instead of building natively
- docker-compose.yml: Add INTERCEPT_IMAGE env var to support pulling
  pre-built images from a registry instead of local build
- README.md: Docker build section rewritten with multi-arch workflow,
  registry pull instructions, and build script options

Weather satellite antenna guide (sidebar panel):
- V-Dipole: ASCII diagram, 53.4cm element length, 120 degree angle,
  materials, orientation, connection instructions
- Turnstile/Crossed Dipole: phasing coax length (37cm RG-58),
  reflector distance (52cm below), RHCP explanation
- QFH Quadrifilar Helix: design overview, materials, height (46cm),
  hemispherical gain pattern
- Placement & LNA: outdoor requirements, coax loss figures,
  LNA mounting position, Nooelec SAWbird+ recommendation, Bias-T
- Quick reference table: wavelength, quarter-wave, elevation,
  duration, polarization, APT/LRPT bandwidth

Also added Weather Satellites and ISS SSTV to README features list,
SatDump to acknowledgments.

https://claude.ai/code/session_01FjLTkyELaqh27U1wEXngFQ

docker-compose.yml

@@ -1,14 +1,18 @@
 # INTERCEPT - Signal Intelligence Platform
 # Docker Compose configuration for easy deployment
 #
-# Basic usage:
-#   docker compose --profile basic up -d
+# Basic usage (build locally):
+#   docker compose --profile basic up -d --build
+#
+# Basic usage (pre-built image from registry):
+#   INTERCEPT_IMAGE=ghcr.io/user/intercept:latest docker compose --profile basic up -d
 #
 # With ADS-B history (Postgres):
 #   docker compose --profile history up -d
 
 services:
   intercept:
+    image: ${INTERCEPT_IMAGE:-}
     build: .
     container_name: intercept
     profiles:
@@ -54,6 +58,7 @@ services:
   # ADS-B history with Postgres persistence
   # Enable with: docker compose --profile history up -d
   intercept-history:
+    image: ${INTERCEPT_IMAGE:-}
     build: .
     container_name: intercept-history
     profiles: